Code Division Multiple Access (CDMA) and Wideband CDMA (WCDMA) receivers in North American bands have been and will continue to coexist with other narrow band systems such as AMPS, IS136, and Global System for Mobile (GSM) communication system. This situation leads to narrow band interference signals that cause both crossmodulation and intermodulation non-linear distortions. The traditional method to mitigate the distortion is to operate with sufficiently high linearity at the expense of additional current drain. Additionally, receiver designs are migrating to higher dynamic range analog-to-digital converters (A/Ds) with digital filters and less analog filtering. This type of receiver design therefore passes both the desired signal and interference through a set of high dynamic range circuit elements before final elimination of the interference by digital filters. Again, the dynamic range of these circuit elements (A/Ds and filters) is set sufficiently high to accommodate the largest expected interference at the expense of current drain and battery life. These two factors cause receivers to operate with higher current drain than needed under the majority of usage conditions.
Interference is a particular problem in CDMA systems which require the receiver and transmitter to be on continuously. An example of such a specification is Telecommunications Industry Association/Electronic Industry Association (TIA/EIA) Interim Standard IS-95, “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (IS-95). IS-95 defines a direct sequence code division multiple access (DS-CDMA or CDMA) radiotelephone system. In a CDMA system, the receiver must be on continuously in order to receive incoming data while on a traffic channel and the transmitter must be on continuously while on a traffic channel.
Prior art receiver linearity systems dynamically adjust the linearity of a receiver based on detection of poor received signal quality based on carrier-to-interference ratio (Ec/Io) or FER (frame error rate), knowledge of the transmit level, and received signal strength indications (RSSI). Also, the prior art captures the various ways that linearity can be changed, i.e. either gain change or current change in receiver amplification stages. However, the prior art does not make the most efficient use of current drain since the linearity is potentially increased without any knowledge of the interference. Poor quality can be caused by a number of factors that are not related to receiver linearity and thus is not the most efficient metric. Furthermore, the prior art does not address the use of variable dynamic range in the baseband.
In another technique, the quality metric shortcoming is somewhat addressed by performing a spectral estimation of the in-band signal. This spectral estimation is performed in post-channel filtering to look for narrowband intermodulation distortion products and a low-noise amplifier (LNA) bypass is used to improve receiver linearity. Note that this addresses potential intermodulation products but does not consider crossmodulation. Also, the mitigation is limited to LNA bypass and thus limits the optimization of performance near sensitivity. Furthermore, detection of the intermodulation products is much more difficult than detection of the interference since the intermodulation products are several dB lower in amplitude than the actual interference.
Dynamic control of A/D converter dynamic range is also known, wherein an analog detected voltage is used to vary an A/D's dynamic range. The variable control of converter range is often internal to the A/D and is not part of a larger system. This limits the efficacy of the techniques to mitigation of narrow-band interferer by the A/D and does not provide a mechanism to reduce the current drain of the other circuit elements, e.g. digital filters, analog filters, and RF circuits.
Accordingly, there is a need for a method and apparatus for reducing current drain in a communication device such as a radiotelephone when mitigating non-linear distortion effects. There is a further need to reduce the current drain by the receiver in a communication device operating in a CDMA system, without sacrificing the ability to receive incoming signals. It would also be of benefit to provide these advantages without additional hardware, which would increase the cost of the communication device.